Conidering that the USSR had a gazillion rights and guarantees on paper, and absolutely none in practice, it's safe to say that when I recommend something, I
never mean something similar to what
they had...
Something much more like this proposal would be far more workable. It would take a lot of tension away, thus weakening the cause of dedicated (willing-to-use-force) separatists. At the same time, it gives the peoples involved enormous control over their own affairs, while preserving the advantages (e.g. shared market, military union, common diplomatic negotiating position) that inclusion within the empire provides.
Note that the map of the proposal reveals that even the proposed internal borders -- while better than what the professional morons carved out post-war in OTL -- are still far from perfect. Therefore, I'd say there are two more considerations if you want to keep violence and other unpleasantries to a minimum:
1) Refine the borders a bit more, using municipality-level census information (and, where doubt persists, a municipal pleibiscite) to make sure the borders reflect the situation on the ground as accurately as possible. If it's all within the same empire anyway, it's not a problem if this produces fairly jagged, HRE-like internal borders.
2) Since people of all ethnicities live all over the empire, you need to give all people who live outside "their own" ethnic homeland a choice: you must register to vote in your own homeland,
or in the region where you live. Either is fine, but you only get to have one voice, and you may choose where to have it heard. So even if you're a Czech living in Vienna, you can still choose to be politically part of the Czech ethnic community, and vote in Czech elections. But this means you
don't get to vote in Viennese elections. And conversely, you can vote in Viennese elections if you want, but then you don't get to vote in Czech elections.
...It may be note that the Ottoman Empire was divided into villayets, but at the same time registered its inhabitants by ethnicity/religion. So that empire, too, is actually well set-up for the sort of system I've just decribed. I think that if this were to be done, both the Austro-Hungarian and the Ottoman empire would have been able to thrive.
Why would I hope for that? Even a peaceful collapse is a mess. If you want to reduce suffering, you should strive to keep the empires together. I get that your OP asks for a monarchy-screw, but this is inherently at odds with the premise of less suffering. The collapse of the monarchies caused a vacuüm of power and legitimacy, and that caused major problems.
If we weigh the benefits of maximised independence versus the benefits of minimised turmoil and suffering, there is little doubt that "highly decentralised multi-ethnic empire" is the best conclusion we may hope for. Even a peaceful split is going to suck for all those Czechs living in Vienna, or the Germans in Hungary, etc. etc.